This invention is directed toward apparatus and methods for obtaining acoustic measurements or xe2x80x9clogsxe2x80x9d of earth formations penetrated by a borehole. More particularly, the invention is directed toward obtaining the acoustic measurements while the borehole is being drilled. In addition, the downhole apparatus comprises a plurality of segmented transmitters and receivers which allows the transmitted acoustic energy to be directionally focused at an angle ranging from essentially 0 degrees to essentially 180 degrees with respect to the axis of the borehole.
Acoustic measurements have been used in wireline borehole logging for the past four decades. The first wireline acoustic instruments or xe2x80x9ctoolsxe2x80x9d were single transmitter and receiver devices which were sed to measure the velocity of the first arrival component of an acoustic wave pulse transmitted through the penetrated formation. This component was usually the compressional or xe2x80x9cPxe2x80x9d wave component. The velocity measurement, or more precisely the travel time of the wave component from the transmitter to the receiver, was used to compute formation porosity in formation evaluation applications. In addition, early acoustic logs were used in the conversion of seismic data, initially measured in the time domain, into the depth domain thereby yielding cross sectional displays of geological structure used in the industry as a guide to exploration and development drilling.
During the late 1960s and early 1970s, acoustic wireline devices became more complex and also yielded additional information. In the area of formation evaluation, multiple transmitters and receivers were introduced to reduce the adverse effects of the borehole upon the formation acoustic measurements. In the late 1970s, as the transmission rates of wireline telemetry systems increased, the full wave form of the received signal, rather than just the first arrival time, was measured at a plurality of receivers spaced axially along the primary axis of the logging tool. The analog signals were digitized downhole and digitized wave forms were transmitted to the surface for processing. Processing involved the extraction of the travel times of the compressional and shear components, as well as various tube wave components. In addition, the amplitudes of the various wave train components were determined. In formation evaluation, the full wave form information was used to obtain a more accurate and precise measure of formation xe2x80x9cacousticxe2x80x9d porosity. In addition, mechanical properties of the formation were determined by combining amplitudes of the various components of the measured acoustic wave form. This information was used to optimize subsequent drilling programs within the area, to aid in the design of hydraulic fracturing programs for the drilled well, and to greatly increase the accuracy and precision of the conversion of area seismic data from the time into the depth domain.
During this same time period, multiple types of logging sensors were beginning to be run in combination, and the measurements from the various types of sensors were combined to obtain formation evaluation information which exceeded the sum of information obtained from the response of each sensor. As an example, thermal neutron porosity sensors, scattered gamma ray sensors, and acoustic sensors were run in combination. Each sensor yielded an indication of formation porosity. By combining the responses of the three types of sensors, a more precise and accurate measure of porosity was obtained. In addition, information concerning the lithology of the formation was obtained which could not be obtained from the responses of any of the individual sensors.
Much effort in the design of acoustic wireline logging tools was, and today still is, directed toward the minimization of acoustic energy transmitted directly through the body of the downhole instrument. The arrival of this energy component at the receiver or receivers usually occurs before the arrival of energy whose path traverses the formation and the borehole. The travel path is more direct and therefore shorter. In addition, the body of the tool is usually metallic and exhibits a faster acoustic travel time than the formation and the borehole. Since the latter arrivals contain parametric information of interest, the former is considered to be interference or xe2x80x9cnoisexe2x80x9d. This direct component is reduced and/or delayed by using a variety of techniques. The component is reduced by acoustically isolating transmitters and receivers from the tool body as much as possible. The arrival of this component is delayed, preferably until after the arrival of components from the formation and borehole, by increasing the effective travel path by cutting a series of alternating slots in the metallic tool body between the transmitter and receiver arrays. This portion of the tool body is commonly referred to as the isolation subsection or xe2x80x9cisolator subxe2x80x9d. In addition, various mathematical techniques have been used in the processing of full wave form data to remove the direct component of the received wave form.
In addition to noise generated by the direct transmission of acoustic energy through the wireline tool body, additional acoustic noise is generated as the tool is conveyed along the borehole wall. This noise is commonly referred to as xe2x80x9croad noisexe2x80x9d. The adverse effects of road noise are minimized using mechanical and mathematical techniques. The prior art teaches the use of many types of roller mechanical devices whereby the wireline tool is xe2x80x9crolledxe2x80x9d rather than xe2x80x9cdraggedxe2x80x9d along the borehole wall thereby reducing the magnitude of the road noise. In addition, since road noise is essentially incoherent, various mathematical methods are used in the processing of full wave form data to greatly reduce the effects of road noise.
The previous discussions have been directed to wireline type measurements wherein the measurements are usually made after the borehole has been drilled. In some drilling operations, wireline logs are made intermittently during the drilling operation, but such logging usually requires that the drill string be removed from the borehole prior to logging. Logging after completion of the drilling operation often reveals that the target formation or formations have been missed by perhaps either drilling too shallow or too deep. In addition, unexpected zones, such as high pressure formations or salt zones, can be encountered during, and adversely affect, the drilling operation. Such encounters can be quite costly and can be fully analyzed with wireline logging only after the encounter. Intermediate logging is likewise costly in that the drilling operation must cease during logging operations. Furthermore, the time interval between the termination of drilling and wireline logging allows the drilling fluid to penetrate or xe2x80x9cinvadexe2x80x9d the near borehole formation thereby possibly introducing error in wireline log measurements. The adverse effects of invasion poses a particularly serious problem for wireline logs with relatively shallow depths of investigation such as most nuclear logs. Possible damage to the borehole can occur during logging and costly drilling rig time and logging equipment time is wasted during stand-by periods for each operation.
Many of the problems discussed above can be overcome by measuring various formation evaluation and other parameters during the actual borehole drilling operation. This is particularly true with acoustic measurements since they not only represent a key formation evaluation measurement but also represent a key seismic tie-in measurement. The problems associated with intermittent logging are essentially eliminated. The need for wireline logging after the drilling can also be eliminated in some cases. Formation evaluation type measurements-while-drilling (MWD) logs can indicate to the driller, in real time, when anomalies such as a fault planes or formation lenses are being penetrated. This is particularly true if the MWD device has a relatively large depth of investigation and if the sensor can make measurements ahead of the drill bit.
Such measurements can also indicate to the driller that high pressure formations or salt zones are bing penetrated thereby allowing time for remedial steps such as adjusting the weight and salinity of the drilling fluid to be made before these zones adversely affect the drilling operation. Real time measures of drilling dynamics data provide the driller with information concerning the efficiency of the drilling operation. Furthermore, borehole directional information combined with real-time formation evaluation parameters, offset wireline log data and possibly seismic data can be extremely useful in assisting the driller in reaching the targeted zones of interest. The measurement-while-drilling (xe2x80x9cMWDxe2x80x9d) acoustic measurement meets, or contributes substantially to, all of the above criteria as will be discussed in the following sections of this disclosure.
The economic, technical, operational and safety advantages of measuring geophysical parameters as well as drilling management parameters, during actual drilling of the borehole were recognized in the early 1950s. Commercial MWD became available in the late 1970s and early 1980s. These measurements included directional information and a limited number of formation evaluation type services. Additional sensors and devices have been added during the intervening time period. In many respects, the sophistication of the sensors are comparable to their wireline counterparts in spite of the harsh conditions experienced in using such sensors in the drilling environment. It is feasible, at least in principle, to utilize multiple sensor combination measurement methods developed for wireline tools to obtain new and improved parametric measurements while drilling. Furthermore, it is feasible, in principle, to utilize additional sensors responding to drilling related parameters simultaneously with formation evaluation type sensors. In practice, however, several major problems exist as will be summarized in the following paragraphs.
Wireline acoustic technology has been particularly difficult to adapt to MWD applications. In addition to road noise generated by the drilling assembly dragging against the wall of the borehole, there is an additional source of noise generated by the rotation of the drill bit and the drill string. Further, the slotted isolation sub technique used to isolate transmitters and receivers in wireline applications can not be used in MWD applications in that such slots would mechanically weaken the MWD acoustic subassembly to the failing point. In addition, the previously described full wave wireline acoustic measurement generates tremendous amounts of digital data. These data exceed the telemetry rates and storage capacities of current MWD systems thereby eliminating the option of processing full wave acoustic data at the surface. This problem is compounded when other types of sensors, comparable in sophistication to corresponding wireline applications, are run in combination with full wave acoustic devices. As an example, it is not feasible using current MWD telemetry capacity to transmit simultaneously a plurality of full acoustic wave forms or gamma ray energy spectra or electromagnetic wave attenuation and phase shift data, or a combination thereof, to the surface for processing to determine parameters of interest at depth intervals sufficient to obtain the required vertical resolution of the penetrated formations. The simultaneous transmission of drilling management sensor information such as directional information, weight on the drill bit, and other non formation evaluation type measurements still further overloads current MWD telemetry transmission rates which are of the order of 2 to 60 bits per second. Furthermore, it is not feasible to store copious amounts of raw data downhole sensor data for subsequent retrieval and processing due to relatively limited storage capacity of current MWD systems. Acoustic and other MWD methods for making multiple formation and borehole evaluation type parametric determinations comparable to current wireline measurements require the computation of the desired parameters downhole, and the transmission of the computed parameters of interest to the surface. By using downhole computational methods, the transmission requirements are reduced by orders of magnitude in that only xe2x80x9canswersxe2x80x9d are telemetered rather than raw data. This type of downhole computation is also applicable to other types of non formation evaluation type measurements such as signals indicative of the operational characteristics of the downhole equipment as well as measurements indicative of drilling direction and efficiency.
The current disclosure is directed toward a full wave acoustic MWD system which utilizes downhole processing to reduce the copious amounts of measured or xe2x80x9crawxe2x80x9d data to parameters of interest, or xe2x80x9canswersxe2x80x9d, which can be telemetered to the surface using current MWD telemetry capacity. The storage capacity of current MWD systems is likewise capable of storing parameters of interest for subsequent retrieval at the surface.
The downhole portion of the acoustic system comprises a plurality fo transmitter sets spaced axially along an essentially tubular downhole subassembly. Each transmitter set further comprises a plurality of segmented transmitters spaced azimuthally around the outside diameter of the downhole subassembly, A plurality of receivers are spaced axially along the outside portion of the downhole subassembly and are separated by an isolation portion of the subassembly such that the direct transmission of acoustic energy through the subassembly is attenuated. One or more of the axially spaced receivers can also comprise an array of receiving elements spaced azimuthally around the outside diameter of the subassembly. The downhole subassembly also comprises a processor, data storage, telemetry elements, power supplies and control circuits as well as other types of sensors. The acoustic portions of the downhole subassembly comprising the acoustic transmitter and receiver arrays will hereafter be referred to as the acoustic subassembly. Drilling fluid is pumped from the surface downward through the drill string, through the acoustic subassembly and any other subassemblies run in combination, through the drill bit, and returned to the surface through the drill string-borehole annulus. The functions and circulation of the drilling fluid or xe2x80x9cmudxe2x80x9d are well known in the art. The design of the isolator portion of the subassembly is such that restrictions to the flow of the drilling mud is minimal.
The physical arrangement and firing sequences of the segmented transmitters are such that acoustic energy can be directed or focused into the formation in a predetermined azimuth and axial direction. This feature of the invention allows acoustic parameters to be measured in selected regions in the vicinity of the downhole assembly Regions to be investigated can be selected in real time using commands from the surface or, alternately, can be preselected. As an example, the segmentation of transmitters allows measurements to be made ahead of the drill bit thereby providing the driller with critical information concerning formations and structures that have not yet been penetrated by the drill bit. This aids the driller in adjusting the drilling program, in real time, to meet the predetermined objectives and avoid problems as discussed briefly in a previous section. As a second example, the circumferential spacing of transmitters permits the focusing of transmitted acoustic energy azimuthally to determine the distance to adjacent bed boundaries in horizontal or highly deviated wells thereby assisting the driller in maintaining the drill bit within the formation of interest. This is referred to as xe2x80x9cgeosteeringxe2x80x9d. Because of the relatively deep depth of investigation of the acoustic measurements, these measurements can be used as reference data for other sensor types with relatively shallow depths of investigation, such as nuclear sensors, run in combination. As an example, acoustic measurements might indicate that the bed boundary of a particular type of formation lies a given distance ahead of the drill bit. This reference information can be used to optimize the response parameters of shallower investigating sensors.
In particular, data processing algorithms for shallow investigating sensors might be adjusted and tailored to yield optimum responses for the particular type of formation which is sensed by the deeper investigating acoustic measurement and will subsequently be penetrated and sensed by the shallow investigating sensors.
Because a plurality of axially and azimuthally spaced sets of segmented transmitters and a plurality of axially spaced receivers are employed, there are multiple paths within the three dimensional space in the vicinity of the device that received acoustic energy can traverse. Some of the receivers can also be segmented thereby further defining the traversion paths. These traversion paths or xe2x80x9cray pathsxe2x80x9d are somewhat analogous to data generated by three dimensional surface seismic source transmitter-receiver arrays or even more analogous to data generated by borehole seismic arrays in which the source is positioned at various positions of the earth and the receivers are positioned at variable depths within one or more boreholes. As in seismology, the full wave acoustic MWD system which will be detailed in this disclosure generates large amounts of raw data due to multiple ray paths and also due to the fact that full wave trains are measured at each receiver. Seismic interpretation techniques, which are available in the art, are suitable for the ray path analysis and interpretation of the MWD data. The processing, however, must be performed downhole since the volume of raw data exceeds existing MWD telemetering and storage capacity. As mentioned previously, the current invention comprises a downhole computer which reduces the raw data to parameter of interest, the volume of which does not exceed current MWD storage and telemetry capacity. Even though downhole processing is provided, parameters of interest must be selected judiciously. As an example, sufficient raw data and sufficient computing power exists to generate a three dimensional map in the vicinity of the drill bit of all geological structures which exhibit an acoustic impedance. It should be recalled that borehole acoustic devices as well as seismic operations respond to changes in acoustic impedance, where acoustic impedance of a material is defined as the product of the density of the material and the velocity of acoustic energy within the material. It would not be possible to telemeter or store a high resolution, three dimensional tabulation of coordinates of the impedance interface surfaces because of limitations of current MWD telemetry systems and storage capacities. It is, however, possible to telemeter or store some information concerning the detected interfaces such as the distance to the nearest interface, coarse coordinates of the interfaces, and the like.
For imaging applications, the segmented transmitter and receiver elements are activated in a sequential manner to provide a xe2x80x9cbeam steeringxe2x80x9d of the transmitted and/or received signals. In another embodiment of the invention, the azimuthally segmented transmitter elements are activated simultaneously. If all the transmitters at a particular axial position are activated in phase with each other, the transmitter acts as a monopole device. By altering the phase of adjacent circumferential transmitter elements, dipole, quadrupole or octupole excitation of the formation is possible. These higher order modes also produce refracted P and S waves. Analysis of the signals received at azimuthally segmented receives provides useful information about the formation.
The parameters of interest that can be provided by the disclosed MWD full wave acoustic system include formation evaluation parameters such as porosity. Additional parameters of interest include Poisson""s ratio, elastic moduli, and other mechanical properties of the formation. In addition, integrated travel times over large vertical intervals can be measured. These parameters of interest have many uses which include detailed formation evaluations by combining acoustic measurements with other types of formation evaluation sensor measurements, pore pressure prediction, reservoir performance predictions, input data for the design of hydraulic fracture operations, input information for the selection of the optimum type of drill bit, geosteering and sand control. Parameters of interest can also be selected to more efficiently convert adjacent surface seismic measurements from the time domain to the desired depth domain.